Cardiopulmonary resuscitation (CPR) is a well-known and valuable method of first aid used to resuscitate people who have suffered from cardiac arrest. CPR requires repetitive chest compressions to squeeze the heart and the thoracic cavity to pump blood through the body. Artificial respiration, such as mouth-to-mouth breathing or a bag mask apparatus, is used to supply air to the lungs. When a first aid provider performs manual chest compression effectively, blood flow in the body is about 25% to 30% of normal blood flow. However, even experienced paramedics cannot maintain adequate chest compressions for more than a few minutes. Hightower, et al., Decay In Quality Of Chest Compressions Over Time, 26 Ann. Emerg. Med. 300 (September 1995). Thus, CPR is not often successful at sustaining or reviving the patient. Nevertheless, if chest compressions could be adequately maintained, then cardiac arrest victims could be sustained for extended periods of time. Occasional reports of extended CPR efforts (45 to 90 minutes) have been reported, with the victims eventually being saved by coronary bypass surgery. See Tovar, et al., Successful Myocardial Revascularization and Neurologic Recovery, 22 Texas Heart J. 271 (1995).
In efforts to provide better blood flow and increase the effectiveness of bystander resuscitation efforts, various mechanical devices have been proposed for performing CPR. In one variation of such devices, a belt is placed around the patient's chest and the belt is used to effect chest compressions. Our own patents, Mollenauer, et al., Resuscitation Device Having A Motor Driven Belt To Constrict/Compress The Chest, U.S. Pat. No. 6,142,962 (Nov. 7, 2000); Sherman, et al., CPR Assist Device with Pressure Bladder Feedback, U.S. Pat. No. 6,616,620 (Sep. 9, 2003); Sherman, et al., Modular CPR assist device, U.S. Pat. No. 6,066,106 (May 23, 2000); and Sherman, et al., Modular CPR assist device, U.S. Pat. No. 6,398,745 (Jun. 4, 2002), show chest compression devices that compress a patient's chest with a belt. Each of these patents is hereby incorporated by reference in their entirety. Our commercial device, sold under the trademark AUTOPULSE®, is described in some detail in our prior patents, including Jensen, Lightweight Electro-Mechanical Chest Compression Device, U.S. Pat. No. 7,347,832 (Mar. 25, 2008) and Quintana, et al., Methods and Devices for Attaching a Belt Cartridge to a Chest Compression Device, U.S. Pat. No. 7,354,407 (Apr. 8, 2008).
These devices have proven to be valuable alternatives to manual CPR, and evidence is mounting that they provide circulation superior to that provided by manual CPR, and also result in higher survival rates for cardiac arrest victims. The AUTOPULSE® CPR devices are intended for use in the field, to treat victims of cardiac arrest during transport to a hospital, where the victims are expected to be treated by extremely well-trained emergency room physicians. The AutoPulse® CPR device is uniquely configured for this use: All the components are stored in a lightweight backboard, about the size of a boogie board, which is easily carried to a patient and slipped underneath the patient's thorax. The important components include a compression belt, motor, drive shaft and drive spool, computer control system and battery.
Addressing another aspect of CPR, chest compression monitoring during the course of CPR is now possible with the Real CPR Help® technology marketed by ZOLL Medical Corporation. This technology is described in U.S. Pat. Nos. 6,390,996, 7,108,665, and 7,429,250, and includes the use of an accelerometer to measure accelerations of the chest and calculating the depth of each compression from the acceleration signal. The technology is used in ZOLL's Real CPR Help® compression depth monitoring system to provide real-time rate and depth CPR feedback for manual CPR providers. Commercially, it is implemented in ZOLL's electrode pads, such as the CPR-D⋅Padz® electrode pads. It is also implemented for training use in the iPhone app PocketCPR®. The same technology can be provided in automatic CPR chest compression devices, such as ZOLL Circulation's AutoPulse® chest compression device, which is described in numerous patents issued to ZOLL Circulation such as U.S. Pat. No. 6,066,106 and its continuations. U.S. Pat. Nos. 6,390,996, 7,108,665, and 7,429,250 also propose use of compression depth monitoring in combination with an automatic constricting device described in U.S. Pat. No. 4,928,674, which is an inflatable vest operable to squeeze the chest of a patient repeatedly to provide CPR chest compressions.
The Real CPR Help® compression depth monitoring system provides valuable unambiguous feedback during manual CPR, because the accelerometer is fixed to the chest of the patient either because is it fixed to electrode pads that are fixed to the patient's chest with adhesive, or because it is fixed relative the CPR providers hands which the CPR provider maintains in the appropriate location over the sternum of the patient. Chest compression information that might be provided during automated CPR with the AutoPulse® device may be unambiguous, assuming that the compression belt used with the AutoPulse® device does not shift during the course of treatment. While this may be monitored visually by an EMT using the AutoPulse®, the system can be improved by providing some mechanism for determining compression depth in the case where the compression belt shifts up or down on the patient's chest during use.
During the course of automated chest compression using the AutoPulse® chest compression device, CPR providers using the device may be concerned about inferior/superior movement of the belt. The device may be operated for several minutes, including time moving the patient into an ambulance, transporting the patient to a hospital, and moving the patient from the ambulance and into a hospital emergency room. With all this movement, it is possible that the compression belt might move either upward toward the patient's shoulders (superiorly, relative to the patient), or downward toward the patient's abdomen (inferiorly, relative to the patient). None of the references discussed above provide a means for detecting horizontal displacement or non-uniformity in the downward movement of a compression component of an automated chest compression device.